7 days is long enough for a seed to sprout and die from damping off or cook from excess heat. We need more info.
What soil?
How old are the seeds?
What kind if lights?
What kind of heat pad?
How deep did you plant the seeds?
What was ambient temps?

Thanks for your response. Sorry it took so long to reply. One of the petunia's is picobella. Purchased this year put under flourescent lights. They might have been too far away from light. The heat pad is for germinating and the temp is about 74. The seeds should have been surface sown but I noticed that the soil washed a little. I will appreciate your response. Marie